Separation of gases via carbonized vinylidene chloride copolymer gas separation membranes and processes therefor

ABSTRACT

A process for separating hydrogen from a gas mixture having hydrogen and a larger gas molecule is comprised of flowing the gas mixture through a carbonized polyvinylidene chloride (PVDC) copolymer membrane having a hydrogen permeance in combination with a hydrogen/methane selectivity, wherein the combination of hydrogen permeance and hydrogen/methane selectivity is (i) at least 30 GPU hydrogen permeance and at least 200 hydrogen/methane selectivity or (ii) at least 10 GPU hydrogen permeance and at least 700 hydrogen/methane selectivity. The carbonized PVDC copolymer may be made by heating and restraining a polyvinylidene chloride copolymer film or hollow fiber having a thickness of 1 micrometer to 250 micrometers to a pretreatment temperature of 100° C. to 180° C. to form a pretreated polyvinylidene chloride copolymer film and then heating and restraining the pretreated polyvinylidene chloride copolymer film to a maximum pyrolysis temperature from 350° C. to 750° C.

The present invention relates to the field of gas separation using acarbon membrane. More particularly, it relates to the separation ofgases and in particular hydrogen from a gas mixture by passing the gasmixture through a carbonized vinylidene chloride copolymer membrane(film or hollow fiber wall).

Carbon molecular sieves (CMS) and CMS membranes have been used toseparate gases. CMSs may be prepared from a variety of resins that arepyrolyzed at various temperatures and/or under various conditions. Thepyrolysis reduces the resins to carbon, but maintains at least someporosity, in the form of micropores, in the pyrolyzed product. The CMSsthus formed may then be employed in conventional gas separationsequipment employing adsorption of particular gases, such as packed beds,columns, and the like, where the micropore size determines which gas ina gas mixture is adsorbed and which is not. Adsorption and desorptiontechniques may be alternated to carry out the separation, according to,for example, conventional pressure swing or temperature swing adsorptionmethods. CMS membranes have also been used to separate gases by flowinggas mixtures through the CMS membranes.

However, there is a particular challenge in the art to prepare CMSshaving micropores of the correct size(s) for certain particularseparations. Since the use of CMSs to accomplish separations assumesthat the micropores are at least as large as, or larger than, thespecified molecule that will enter the micropores, it is necessary toknow the “size” of the molecule. Different ways to determine thatmolecular size have been developed. One commonly employed approach hasbeen to determine a given molecule's “kinetic diameter.” A referencelisting a variety of these kinetic diameters, based upon their use inzeolite applications, is D. W. Breck, Zeolite Molecular Sieves:Structure, Chemistry and Use, John Wiley & Sons, Inc. (New York, N.Y.1974), 636, and these determinations are frequently used even withrespect to non-zeolite, carbon molecular sieves that are known to haveslit-shaped pores. In view of the above and for purposes hereof, then,the following kinetic diameters, taken from the Breck reference citedsupra, are used herein as the representative molecular diameters for thefollowing molecules: He (2.6 Angstroms, Å), H₂ (2.89 Å), N₂ (3.64 Å),CO₂ (3.3 Å), CH₄ (3.8 Å), C₂H₄ (3.9 Å), C₃H₈ (4.3 Å), i-C₄H₁₀ (5.0 Å),SF₆ (sulfur hexafluoride) (5.5 Å), and i-C₈H₁₈ (iso-octane) (6.2 Å).However, because that reference table lacks a kinetic diameter forethane, and the kinetic diameter given therein for propylene is believedby at least some researchers to be inaccurate for CMS materials per se,the Lennard-Jones collision diameters are used herein, instead of theBreck kinetic diameters, for those two materials. These Lennard-Jonescollision diameters are, respectively, C₂H₆ (4.1 Å), and C₃H₆ (4.0 Å).See, for example, Staudt-Bickel C., Koros W. J., “Olefin/paraffin gasseparations with 6FDA-based polyimide membranes,” J. Membr. Sci. (2000)170 (2), 205-214 for further discussion. The kinetic diameters andLennard-Jones collision diameters are referred to together as“representative molecular diameters.”

Polyvinylidine chloride (PVDC) copolymers have been pyrolyzed to formcarbon molecular sieves, but they have tended to form larger pores.Lamond T. G., et al., “6 Å molecular sieve properties of SARAN-typecarbons,” Carbon (1965) 3, 59-63. This article describes preparation ofa CMS, from a polyvinylidene chloride (PVDC) copolymer, that rejectsneopentane (6.0 Å) molecules, but adsorbs smaller molecules, such as, innon-limiting example, CO₂, butane, and iso-butane, non-selectively. Inview of this the authors of that article concluded that their CMS had 6Å micropores.

Another example is disclosed in T. A. Centeno., et al., “Molecular sievegas separation membranes based on poly(vinylidene chloride-co-vinylchloride),” Carbon (2000) 38, 1067-1073. This article describespreparation of a composite carbon membrane using the aforesaid material.The membrane is formed with a thin microporous carbon layer (thicknessof 0.8 micrometers, μm) obtained by pyrolysis of the polymeric film,supported over a macroporous carbon substrate (pore size 1 μm;macroporosity 30 percent, %). Single gas permeation experiments includehelium (He), CO₂, oxygen (O₂), nitrogen (N₂) and methane (CH₄).Selectivities are described as particularly high for O₂/N₂ systems,i.e., a selectivity of about 14 at 25 degrees Celsius (° C.). From thisinformation it can be inferred that the micropore size falls somewherein a range from the representative molecular diameter of O₂ (3.46 Å) tothat of N₂ (3.64 Å). This CMS membrane is prepared by pre-treating thesupported film at 200° C., a temperature at which the PVDC copolymerprecursor is melted before carbonization. The fact that melting isrequired means that the disclosed CMS structures cannot be prepared inunsupported forms.

In other research, including for example, Laredo G. C., Meneses E.,Castillo J., Marroquin J. O., Jimeenez-Cruz F., “Adsorption equilibriumand kinetics of branched octane isomers on a polyvinylidenechloride-based carbon molecular sieve,” Energy Fuels (2008) 22 (4)2641-2648, polyvinylidene chloride copolymer-based CMSs have beenprepared that exhibit relatively large micropore sizes and pore volumesthat are suitable for separation of correspondingly large molecules,i.e., those having a representative molecular diameter greater than 5.0Å.

More recently, WO/2016/003680 described forming a CMS from PVDCcopolymers using a two-step pyrolysis at high temperatures from 800° C.to 1700° C. The CMS formed had an average pore size in the range of 3 Åto 5 Å. These CMS were described as being useful for separatingPropylene (C₃H₆) and propane (C₃H₈); carbon dioxide (CO₂) and nitrogen(N₂); N₂ and methane (CH₄); ethylene (C₂H₄) and ethane (C₂H₆); andn-butane (C₄H₁₀) and i-butane (C₄H₁₀).

CMS membranes formed from PVDC copolymers have also been made, but theyhave suffered from low or reverse (rejection of hydrogen with presenceof hydrocarbons) hydrogen selectivity as described by M. B. Rao and S.Sircar in J. Membrane Science, 85 (1993) 253-264; T. A. Centeno and A. BFuertes in Carbon, 38 (2000) 1067-1073 and K. Zhang and J. D. Way in J.Membrane Science 369(2011)243-249.

The gas permeation properties of a membrane can be determined by gaspermeation experiments. Two intrinsic properties have utility inevaluating separation performance of a membrane material: its“permeability,” a measure of the membrane's intrinsic productivity; andits “selectivity,” a measure of the membrane's separation efficiency.One typically determines “permeability” in Barrer (1 Barrer=10⁻¹⁰ [cm³(STP) cm]/[cm² s cmHg], calculated as the flux (n_(i)) divided by thepartial pressure difference between the membrane upstream and downstream(Δp_(i)), and multiplied by the thickness of the membrane (l).

$P_{i} = \frac{n_{i}l}{\Delta p_{i}}$

Another term, “permeance,” is defined herein as productivity of themembrane or hollow fiber membranes and is typically measured in GasPermeation Units (GPU) (1 GPU=10⁻⁶ [cm³ (STP)]/[cm² s cmHg]), determinedby dividing permeability by effective membrane separation layerthickness.

$( \frac{P_{i}}{l} ) = \frac{n_{i}}{\Delta p_{i}}$

Finally, “selectivity” is defined herein as the ratio of one gas'spermeability through the membrane or permeance relative to the sameproperty of another gas. It is measured as a unitless ratio.

$\propto_{i/j}{= {\frac{P_{i}}{P_{j}} = \frac{( {P_{i}/l} )}{( {P_{j}/l} )}}}$

Thus, it would be desirable to realize a CMS membrane and process tomake the CMS membrane from PVDC that would be useful in separatinghydrogen from gas mixtures such as those encountered in syngas, gasesgenerated in oil refineries, natural gas and olefin cracker gas streams.It would be particularly desirable to provide a PVDC CMS that is in theform of an un-supported membrane or hollow fiber.

One aspect of the invention is a process for separating hydrogen from agas mixture having hydrogen and a larger gas molecule, the methodcomprising

-   -   (i) providing a carbonized polyvinylidene chloride copolymer        membrane having a hydrogen permeance in combination with a        hydrogen/methane selectivity, wherein the combination of        hydrogen permeance and hydrogen/methane selectivity is (i) at        least 30 GPU hydrogen permeance and at least 200        hydrogen/methane selectivity or (ii) at least 10 GPU hydrogen        permeance and at least 700 hydrogen/methane selectivity; and    -   (ii) flowing the gas mixture through said carbonized        polyvinylidene chloride copolymer membrane to produce a first        permeate stream having an increased concentration of the        hydrogen and a second retenate stream having an increased        concentration of the larger gas molecule.

A second aspect of the invention is a method of making a carbonizedpolyvinylidene chloride copolymer comprising,

-   -   (a) providing a polyvinylidene chloride copolymer film or hollow        fiber having a thickness of 1 micrometer to 250 micrometers,    -   (b) heating and restraining the polyvinylidene chloride        copolymer film to a pretreatment temperature of 100° C. to        180° C. to form a pretreated polyvinylidene chloride copolymer        film,    -   (c) heating and restraining the pretreated polyvinylidene        chloride copolymer film to a maximum pyrolysis temperature from        350° C. to 750° C. Applicants have surprisingly discovered a        PVDC CMS membrane that even though it has an average or        representative pore size significantly larger than hydrogen and        even larger than the gas molecule to be separated (e.g.,        methane) as determined by adsorption of gas molecules the PVDC        CMS membrane has a high hydrogen permeance yet a high        selectivity in separating the hydrogen from gas molecules such        as methane (e.g., H₂/CH₄ selectivity). It is unknown why, but        without being limiting in any way, it is believed that the        particular process may realize an asymmetric microstructure        across the film thickness where there may be large pores at the        surface of the film and a narrow band within the film that has a        smaller pore size. The asymmetry may be a result of varying        localized atmospheres during the pre-treatment and pyrolysis        (e.g., partial pressure of HCl), which may be due to the        thickness of the films, restraining of the films or combinations        thereof.

The PVDC CMSs of the invention may be prepared from a vinylidenechloride copolymer, comprising a vinylidene chloride monomer and atleast one additional comonomer. The comonomer may be selected from avariety of materials, including in particular embodiments a vinylmonomer, vinyl chloride monomer, an acrylate monomer, a methacrylatemonomer, a styrenic monomer, acrylonitrile, methacrylonitrile, itaconicacid, chlorotrifluoroethylene, and combinations thereof. In moreparticular embodiments examples of the vinyl monomers include vinylchloride, vinyl acetate, acrylonitrile, and combinations thereof. Moreparticular examples of the acrylate monomers include methyl acrylate,ethyl acrylate, butyl acrylate, and combinations thereof. Moreparticular examples of methacrylate monomers include methylmethacrylate, butyl methacrylate, and combinations thereof. A moreparticular example of styrenic monomers is styrene itself.

In proportion it is preferred that the vinylidene chloride basedcopolymer, which is herein termed a polyvinylidene copolymer (PVDC),includes at least 60 wt % of vinylidene chloride, based on total weightof the copolymer, and in more preferred embodiments at least 70 wt %.However, it is further desired that the PVDC contains a maximum of 97 wt% vinylidene chloride, and thus preferably contains a minimum of atleast 3 wt % of the comonomer or comonomer combination; more preferablyfrom 3 wt % to 40 wt %; still more preferably from 3 wt % to 30 wt %;and most preferably from 3 wt % to 20 wt %.

Particular embodiments of PVDCs that are suitable for use in theinvention are those including as a comonomer an acrylate, such as methylacrylate, ethyl acrylate, butyl acrylate, or a combination thereof, inan amount from 3 wt % to 20 wt %, based on the weight of the PVDC as awhole; more preferably from 3.5 wt % to 15 wt %; and most preferablyfrom 4 wt % to 12 wt %. Another particular embodiment is a PVDCincluding vinyl chloride in an amount from 3 wt % to 30 wt %; morepreferably from 7 wt % to 28 wt %; and most preferably from 9 wt % to 25wt %.

It is also preferred that the overall weight average molecular weight(Mw) of the PVDC copolymer ranges from 10,000 to 250,000; morepreferably from 50,000 to 200,000; and most preferably from 60,000 to150,000.

Use of additives in the PVDC is also contemplated as being within thescope of the invention. Common additives may include, but are notnecessarily limited to, epoxidized oil stabilizers such as expoxidizedsoybean oil, expodized linseed oil, and the diglycidyl ether ofbisphenol A. Also frequently employed are liquid plasticizers such asaliphatic and aromatic esters, including for example dibutyl sebacate,acetyl tributyl citrate, dioctyl phthalate, and the like, andcombinations thereof. Other common additives may include lubricants,such as polyethylene wax, paraffin wax, oxidized polyethylene wax, andcombinations thereof. Lubricants may optionally be included, and maycomprise, for example, high density polyethylene, acrylate copolymersand silicone polymers, and combinations thereof. Another group ofadditives that may be included are acid scavengers such as epoxycompounds, magnesium hydroxide, magnesium oxide, tetrasodiumpyrophosphate, calcium phosphate, magnesium phosphate, DHT 4A (asynthetic hydrotalcite-like halogen scavenger available from KyowaChemical Industry), calcium oxide, calcium carbonate, and combinationsthereof. Antioxidants such as phenolics may also be incorporated.Combinations of any or all of these types of additives may be includedin the PVDC.

In proportion, it is preferred that the total amount of all additivescombined be no more than 15 wt %, and more preferably no more than 8 wt% or 3 wt %. In many applications, however, an amount of all additivescombined of at least 2 wt % may be typical, with use thereof thereforeranging preferably from 2 wt % to 8 wt %, and more preferably from 2 wt% to 3 wt %. Those skilled in the art will be aware of the use of suchadditives and their indications and contraindications without furtherdirection herein.

Those skilled in the art will also be aware of a variety of means andmethods for preparing copolymers. However, in general any of the typicalor conventional methods of polymerization, including but not limited tomass polymerization, suspension polymerization, and emulsionpolymerization, and preferably suspension polymerization or emulsionpolymerization, may be employed. It is generally preferred thatpolymerization is carried out at a temperature that ensures avoidance ofdegradation of all of the PVDC components, e.g., preferably from 10° C.to 120° C.; more preferably from 20° C. to 100° C.; and most preferablyfrom 30° C. to 90° C.

Following completion of the copolymerization, the PVDC may be formedinto a film or hollow fiber by any suitable method such as those knownin the art. For example the PVDC may be melt-extruded, solution or latexcast in order to form the PVDC into a thin film or hollow fiber. Wherefilms are desired, a conventionally known preparation process such as ablown film process, for example, a double bubble process or a cast filmtentering process, may be especially useful to produce a biaxiallyoriented film. It is more preferred that a double bubble process beemployed in order to concurrently extrude, biaxially orient, and annealthe PVDC film. Fibers may be produced by uniaxial stretching using knownfiber processes for PVDC copolymers, and may be round or shaped hollowfibers, or of any other desired hollow fiber morphology. It is alsocontemplated that precursor films and/or fibers may be coextruded withmultiple PVDC copolymers and/or with other polymers.

It is noted that either the film or fiber preparation process mayoptionally include stretching, such as stretching of the resin to form amelt-extruded film or fiber. This stretching may, in particularembodiments, be particularly effective in inducing more rapidcrystallization and in increasing, and therefore improving, alignment ofthe PVDC crystallites. Desirably the stretch ratio ranges from 1 to 8,more desirably from 1 to 6, still more desirably from 1 to 4, and mostdesirably from 2 to 4.

Generally it is useful for the PVDC to have some amount ofcrystallinity. In the present invention this crystallinity typicallyranges from 25% to 75% of the resin or formed film, as measured bydifferential scanning calorimetry (DSC) according to ASTM D3418. It ismore preferred that this level ranges from 30% to 55%, and mostpreferred that this level ranges from 35% to 50%. Thus, inclusion of acomonomer generally helps to reduce precursor crystallinity to ensurethe desired range, and also to reduce the melt temperature and therebyimprove processability of the resulting copolymer. In general, inclusionof bulkier monomers may tend to reduce overall copolymer crystallinityby a greater amount than inclusion of less bulky monomers. Thus, forexample, butyl acrylate will tend to reduce crystallinity more than, forexample, methyl acrylate or ethyl acrylate, assuming such is/are used inthe same mole percent (mol %) based on final copolymer composition.

To form the PVDC CMS films or hollow fibers of the present invention apre-treatment prior to pyrolysis is employed. Generally thepre-treatment is used to stabilize, or “lock,” the copolymer structureprior to carbonization thereof. In this step the PVDC film or fiber areheated, below the melting temperature thereof (typically less than about180° C., depending upon the exact composition of the precursor), inorder to dehydrochlorinate the film to the extent of at least 10%. Asused herein, the term “at least 10% dehydrochlorinated” means that thefilm or fiber has been pre-treated, by removing hydrogen chloride, to apoint at which the PVDC copolymer film or fiber no longer melts and, infact, begins to become infusible. It is well-accepted in the art thatsuch a change in molecular kinetics begins to occur at a point ofapproximately 10% dehydrochlorination and is completed or maintained asthe level of dehydrochlorination increases above that point. This stepis termed a “pre-treatment” because it occurs prior to a pyrolysis step,which is the treatment step wherein carbonization is accomplished.

During the pre-treatment the copolymer structure's temperature ispreferably maintained in a range of from 100° C. to 180° C., morepreferably from 120° C. to 160° C., and most preferably from 130° C. to150° C. This is preferably done in air for convenience, but otheratmospheres, such as N₂ and other inert gases or oxidizing gases such asCO₂, or combinations thereof, may also or alternatively be used, sincegenerally only minor levels of oxidation of the copolymer areanticipated within the overall given temperature range. Achievement ofthe desired dehydrochlorination, that is responsible for the formationof the locked structure, may be accomplished by exposure to a source ofhigh energy irradiation, such as gamma rays, an electron beam,ultraviolet light, or a combination thereof. The time may vary from 1hour (hr) to 48 hr, preferably from 1 hr to 24 hr, and most preferablyfrom 1 hr to 12 hr, as needed to reach the at least 10%dehydrochlorination point, at which the copolymer begins to becomeinfusible, i.e., no longer able to be melted. The dehydrochlorinationdegree can vary from 5% to 100%, depending upon pretreatment temperatureand time. Where more than visual confirmation of the beginning ofinfusibility is desired, additional confirmation of the percentage ofdehydrochlorination may be obtained by means of, for example, ThermoGravimetric Analysis (TGA), using standard and well-known methods andequipment.

During the pre-treatment the fiber or film is restrained to maintain itsshape. The particular restraining method may be any known in the art andmay be held in tension or compression. In a particular embodiment,particularly for films, they are restrained by applying a compressiveforce. In particular the film is placed between two flat substrates thatmay be impervious to gases including the HCl being removed.Illustratively, the film may be constrained between two low surfaceenergy plates (e.g., TEFLON plates or sheets), which are furtherinterposed between two metal, ceramic or graphite plates. Alternatively,the plates may be pervious to gases such as the HCl being removed suchas honeycomb structures. The amount of tension or compression may be anyuseful amount, but typically may range from 0.01 MPa to 10 MPa, from 0.1to 1 MPa, or from 0.1 to 0.5 MPa. In the same manner, the restrainingduring pyrolysis may be performed in the same fashion with similarsubstrates, which can withstand the maximum pyrolysis temperatures used.

Following the dehydrochlorination pre-treatment, the pre-treated film orpre-treated fiber, or alternatively pre-treated CMS material, ispyrolyzed. Preferably such pyrolysis results in at least 90 wt % of thecopolymer becoming carbonized, more preferably at least 95 wt %, andmost preferably at least 99 wt %. As already pointed out hereinabove,this pyrolysis is also termed “carbonization,” because the resultthereof is that the copolymer is converted to the carbon-only, or nearcarbon-only, skeleton of its copolymer structure, i.e., all or virtuallyall atoms other than carbon have been removed, but the carbon-carbonbonds remain substantially intact, and the CMS may now be termed to be“carbonaceous.” The pyrolysis may be carried out using any meansgenerally known to those skilled in the art, but may be carried out atan attained maximum temperature within the range of from 350° C. to 750°C. Desirably, the temperature is at least 400° C., 450° C. to at most700° C., 650° C., 600° C. or 550° C.

The method may form a PVDC CMS membrane that has a combination ofhydrogen permeance and hydrogen/methane selectivity highly useful forseparating hydrogen from gases containing methane. Such combinationsgenerally require reasonably high hydrogen permeance with reasonablyhigh selectivity or lower hydrogen permeance with much higherselectivity. Generally, if the hydrogen permeance is less than 30 GPU,it is desirable for the hydrogen/methane selectivity to be at least 700.If the hydrogen permeance is greater than 30 GPU, the hydrogen/methaneselectivity may be about 200 or greater. Regardless, it is desirable forthe hydrogen permeance to be at least 40, 50, 60, 80 or even 100 toseveral hundreds. Desirably, the hydrogen/methane selectivity is atleast 100, 150, 200, 250, 400 or even 500 to any practical amount (e.g.,several thousand) when the hydrogen permeance is greater than about 30.Desirably, the hydrogen/methane selectivity is at least 750, 800, 900,or even 1000 to any practical amount (e.g., several thousand) when thehydrogen permeance is less than 30 (i.e., about 10 to 30).

Surprisingly, the PVDC membranes may have an average pore size that islarger than hydrogen and a larger gas molecule than hydrogen in a gasmixture. The larger gas molecule may be comprised of olefins orparaffins. Examples of the larger gas molecule include carbon dioxide,nitrogen, carbon monoxide, methane, ethane, propane, ethylene,propylene, butane, or butylene. It is surprising, since the selectivityof hydrogen/methane as well as for other hydrogen/larger gas moleculeselectivities, even though the average pore size of the membrane islarger than the larger gas molecule diameter, which would indicate thatsuch larger gas molecule would not be preferentially rejected whenflowing through the membrane (i.e., fits into the pores and would beexpected to flow through the membrane). Thus, this gives rise to thebelief an asymmetric structure is formed in the film as described above.In general, the average pore size of the PVDC CMS membrane at least 3 Å,4 Å or even 5 Å to at most about 15 Å. The average pore size of themembrane may be determined by adsorption as described below in theExamples.

In addition to average micropore size, it is also often desirable in theart to optimize total micropore volume, which may be measured via theBrunauer-Emmett-Teller (BET) method at liquid N₂ temperature. Such maybe further confirmed via helium (He) pycnometry and mercury (Hg)intrusion. For most separations applications, a total micropore volumeof at least 0.10 mL/g, preferably at least 0.15 mL/g, more preferably atleast 0.20 mL/g, according to the BET method at liquid N₂ temperature,is needed to ensure commercially efficient desirable gas adsorption.

The average micropore size and/or average micropore volume seem tosuffer little, if any, alteration when additional factors, including butnot limited to ramp rate to reach the attained maximum pyrolysistemperature, and/or hold time at the attained maximum pyrolysistemperature, are introduced and/or considered. For example, forindustrial purposes, ramp rates ranging from 0.1° C./min to 10° C./minare typical, and hold times may range from 0 minutes (min) (i.e.,ramping to the attained maximum temperature followed by immediate activeor passive temperature reduction) up to 60 min (i.e., holding at theattained maximum pyrolysis temperature for up to 60 min prior to activeor passive temperature reduction) are typical. The atmosphere may be anythat realizes the PVDC CMS membrane. That is the atmosphere that doesnot substantially oxidize the PVDC and may include inert, or reducingatmospheres such as static or flowing nitrogen, inert gas (e.g., argon),carbon monoxide, hydrogen or any combination thereof.

The CMS membranes are particularly suitable for separating hydrogen fromother gases in a gas feed containing hydrogen and another larger gasmolecule. In performing the process, the gas feed is flowed (over andthrough the membrane) such that a first stream (permeate) having anincreased concentration of the desired hydrogen and second stream(retentate) having an increased concentration of the other gasmolecule(s) results. When practicing the process, the CMS membrane isgenerally fabricated into a module comprising a sealable enclosurecomprised of one or more of the carbon molecular sieve membranesdescribed herein contained within the sealable enclosure. The sealableenclosure has an inlet for introducing a gas feed comprised of hydrogenand at least one other larger gas molecule; a first outlet forpermitting egress of a permeate gas stream (i.e., hydrogen); and asecond outlet for egress of a retentate gas stream (i.e., the otherlarger gas molecule(s)).

EXAMPLES PVDC Copolymer Film Preparation: Melt Extruded Films of MA 4.8wt % Copolymer

Base resin XUS32904.01 available from The Dow Chemical Company, Midland,Mich. (PVDC copolymer with 4.8 wt % methyl acrylate (MA) comonomer,Mw=96,000) was blended with 2 wt % epoxidized soybean oil (based ontotal amount of blend), 4 wt % dibutyl sebacate, and 2 wt %PLASTISTRENGTH L-1000 an acrylic lubricant available from Arkema PLC,France. The blend was extruded through a 1.75 inch width film die(controlled at 174° C.) followed by water quench and stretch winding.The wind rate was controlled to obtain films of different thicknesses:2, 4, 8, and 12 mil (1 mil=25.4 micrometer). The films after windingwere cut into approximately 12 inch wide and 2 feet length pieces andlaid on flat desktop for about one week. Coupons of ⅞ inch diameter werecut for carbonization as described below.

Melt Extruded Films of MA 8.5 wt % Copolymer

Base resin SARAN 806 available from The Dow Chemical Company (PVDCcopolymer with 8.5 wt % methyl acrylate comonomer, Mw=85,000) wasblended with 2 wt % epoxidized soybean oil and 2 wt % PLASTISTRENGTHL-1000. The blend was extruded in the same manner as above. The windrate was controlled to obtain films of different thicknesses: 2, 4, 8,and 12 mil. The films after winding were cut into approximately 12 inchwide and 2 feet length pieces and laid on flat desktop for about oneweek.

Solution Cast Films of MA 4.8% Resin

Base resin XUS 32904.01 was dissolved in tetrahydrofuran (THF) torealize a 15 wt % polymer solution. The solution was poured onto a flatglass plate and cast using a knife having a 28 mil clearance.Approximately 2 mil films were obtained after the THF evaporated in air.

Solution Cast Films of VC 17.6% Resin

Base resin XUS 32061.01 was dissolved in tetrahydrofuran to make a 15 wt% polymer solution. The solution is poured onto a flat glass plate andcast using a 28 mil clearance knife. Approximately 2 mil film wasobtained after solvent evaporation.

Latex Cast Films of DARAN SL158

Latex dispersion of DARAN SL158 (Owensboro Specialty Polymers, Inc.) waspoured onto glass plate and cast using a knife having a 4 mil clearance.The cast films were dried at 75° C. in an air purged oven for about 2hours. Approximately 1.5 mil films were obtained. Table 1 shows asummary of information on all eleven precursor films.

TABLE 1 Precursor Films Film Precursor thickness Film # Preparationmethod Base resin [mil] 1 Melt extrusion XUS32904.01 2 2 Melt extrusionXUS32904.01 4 3 Melt extrusion XUS32904.01 8 4 Melt extrusionXUS32904.01 12 5 Melt extrusion SARAN 806 2 6 Melt extrusion SARAN 806 47 Melt extrusion SARAN 806 8 8 Melt extrusion SARAN 806 12 9 Solutioncasting XUS 32904.01 2 10 Solution casting XUS 32061.01 2 11 Latexcasting DARAN SL158 1.5

Carbon Membrane Formation

A two-step pyrolysis approach was used. The precursor films were heatedto a first temperature of 130-150° C. for 24 hours in a low temperatureoven purged by 2 L/min of air (pretreated films), which was followed byfurther heating to pyrolyze the pretreated films to temperatures in therange of 350-950° C. in a 6″ ID quartz tube furnace purged by 5 L/min ofnitrogen.

For the initial low temperature pretreatment, 12 disks (⅞ inch diameter)sandwiched between graphite plates, with two pieces of 10 mil TEFLONsheets being used to separate the two sides of the membrane from thegraphite plates. The weight of graphite plates are about 0.2-0.8 kg.Alternatively, the graphite plates and TEFLON sheets were replaced withporous ceramic honeycomb plates, through which HCl generated should betransported out swiftly. A scrubber connected to this oven contained a10 wt % sodium hydroxide aqueous solution. A loaded oven was heated at1° C./min to 130, 140, or 150° C. and held for 24 hour under 2 L/min ofair purge.

For the second heating step, the 12 pretreated disks were sandwichedbetween the graphite plates without the Teflon sheets or honeycombplates were loaded into a 6″ ID quartz tube furnace. A scrubberconnected to this furnace contained a 10 wt % sodium hydroxide aqueoussolution. The furnace was raised to different final temperatures rangingfrom 350-950° C. at various ramp rates (1, 3, 5° C./min), and held for30 minutes at the final temperature and then cooled down to roomtemperature (˜25° C.). After cooling down, the carbon membranes were putinto a storage box continuously purged with dry nitrogen at a flow rateof 5 Liter/min.

Carbon Membrane Permeation Test Protocol

The carbon membranes were masked onto a standard 25 mm filter holder(Millipore #4502500, EMD Millipore Corp., Germany) using an impermeablealuminum tape, leaving an open defined permeation area. A two-part epoxy(J-B Weld twin tube) was then applied along the interface of the tapeand the carbon membranes. Single gas permeation tests of several gasspecies were conducted at 20° C. with a continuous upstream feed (25sccm, 1 atm) and downstream He purge (2.0 sccm, 1 atm). The permeatecarried by the He purge gas was analyzed by a GC (gas chromatograph)with a TCD (thermal conductivity detector for H₂ and CO₂) and FID (flameionization detector for CH₄). The concentrations in all gases were lowerthan 5%, so the gas flow rate in downstream was considered the same asthe He flow rate. The membrane permeate rate was calculated using the Hepurge flow rate times the permeate concentrations measured by GC. Thesingle gas permeation tests were conducted in the following order—H₂,CO₂, and CH₄. The tests were run for several hours to days until thepermeate concentrations were steady. The parameters to make the carbonmembranes and the resulting permeation results are shown in Table 2.

TABLE 2 Final Pyrolysis Hydrogen Methane CMS film Pre-treatment Temp.Ramp rate Precursor Sandwich Permeance Permeance H₂/CH₄ Example Temp. [°C.] [° C.]* [° C./-min] film # Plates (GPU) (GPU) Selectivity Comp Ex 1140 350 1 3 Graphite 0.7 0.026  28 Ex 1 130 950 1 4 Graphite 11.1 0.003*4269* Ex 2 130 650 1 1 Graphite 53.0 0.058 914 Ex 3 140 950 3 2 Graphite10.7 0.008* 1390* Comp Ex 2 150 350 3 1 Graphite 3.7 0.083  45 Ex 4 150800 1 2 Graphite 17.5 0.017 1007  Ex 5 150 650 3 2 Graphite 49.5 0.064773 Ex 6 130 800 3 4 Graphite 17.3 0.018 979 Ex 7 130 500 3 3 Graphite65.4 0.274 239 Ex 8 140 650 5 4 Graphite 37.6 0.044 863 Comp Ex 3 150500 1 4 Graphite 37.4 0.105 356 Comp Ex 4 130 350 5 2 Graphite 1.3 0.049 27 Ex 9 140 500 5 1 Graphite 35.0 0.017 2120  Ex 10 150 950 5 3Graphite 19.8 0.011 1850  Ex 11 130 500 3 1 Honeycomb 169.0 0.487 350 Ex12 130 500 3 2 Honeycomb 96.9 0.479 246 Ex 13 130 500 3 3 Honeycomb 66.40.483 149 Ex 14 130 500 3 4 Honeycomb 46.2 0.257 204 Ex 15 130 500 3 5Honeycomb 151.8 1.598  95 Ex 16 130 500 3 6 Honeycomb 92.3 0.322 286 Ex17 130 500 3 7 Honeycomb 65.7 0.614 107 Ex 18 130 500 3 8 Honeycomb 47.70.171 279 Ex 19 130 500 3 9 Honeycomb 90.5 0.135 670 Ex 20 130 500 3 10Honeycomb 125.5 0.756 166 Ex 21 130 500 3 11 Honeycomb 98.6 0.206 478 Ex22** 150 700 1 2 Honeycomb 78.0 0.26 385 *Very close to detection limit**Pretreatment for 150° C. for 60 hours, then pyrolysis with 1° C./minto 700° C., hold 0 minute

From the results shown in Table 2, the final pyrolysis temperature hasthe most effect on the H₂ permeance and H₂/CH₄ selectivity. Themembranes with final temperature around 500-650° C. showed the bestcombination of H₂ permeance and H₂/CH₄ selectivities. In Examples 11-14,which were made of different thicknesses of melt extruded PVDC-MA 4.8%resin, both H₂ permeance and H₂/CH₄ selectivity increase as theprecursor film thickness decreases from 12 mil to 2 mil. Examples 15-18,which were made of different thicknesses of melt extruded PVDC-MA 8.5%resin, the H₂ permeance increases continuously as the precursor filmthickness decreases from 12 mil to 2 mil, while the H₂/CH₄ selectivitypeaks at 4 mil. Therefore, the optimum thickness of precursor film forthe combination of H₂ permeance and H₂/CH₄ selectivity is somewhatdependent on the copolymer composition.

Examples 19 and 20 (solution cast precursor films) as well as Example 21(latex precursor film) show similar high H₂ permeance and H₂/CH₄selectivities as those made of melt extruded films.

Carbon Membrane Adsorption

Gas adsorption was used to measure the average pore size of Example 13.The adsorption was performed using a Micromeritics ASAP 2020 instrumentat 20° C. Carbon membranes were broken into pieces (˜2-5 mm) and loadedinto a quartz sample holder. Each sample was degassed at 100° C. for 12hours before each gas adsorption was performed in the sequence of CH₄,CO₂, C₂H₄, C₃H₆, and iC₄H₁₀.

All the gases, except iC₄H₁₀, adsorb in large amounts (greater thanabout 10 cc (STD)/g) at ˜600 mmHg: C₃H₆ (83.3 cc(STD)/g) and iC₄H₁₀ (4.1cc(STD)/g). From these results, the average micropore size wasconsidered to be between the molecular size of C₃H₆ and iC₄H₁₀, 4.0-5.0Å. Therefore, it is expected that the Example 13 carbon membrane wouldpermeate CH₄ (3.8 Å molecular size) at a high rate. However,surprisingly, it was found that CH₄ gas essentially does not permeatethrough the Example 13 carbon membrane.

1.-15. (canceled)
 16. A method of making a carbonized polyvinylidenechloride copolymer comprising, (a) providing a polyvinylidene chloridecopolymer film or hollow fiber having a thickness of 1 micrometer to 250micrometers, (b) heating and restraining the polyvinylidene chloridecopolymer film to a pretreatment temperature of 100° C. to 180° C. toform a pretreated polyvinylidene chloride copolymer film, and (c)heating and restraining the pretreated polyvinylidene chloride copolymerfilm to a maximum pyrolysis temperature from 350° C. to 750° C.
 17. Themethod of claim 16, wherein the restraining of steps (b) and (c) is byapplying a compressive force.
 18. The method of claim 16 or 17, whereinthe maximum pyrolysis temperature is at most 650° C.
 19. The method ofclaim 16, wherein the polyvinylidene chloride copolymer film is apolyvinylidene chloride copolymer comprised of vinylidene chloride andat least one of the following: a vinyl monomer; a vinyl chloridemonomer; an acrylate monomer; a methacrylate monomer; a styrenicmonomer; acrylonitrile, methacrylonitrile; itaconic acid;chlorotrifluoroethylene that have been copolymerized.
 20. The method ofclaim 16, wherein the polyvinylidene chloride copolymer has acrystallinity percentage of 25% to 75% determined by differentialscanning calorimetry.
 21. The method of claim 16, wherein thepolyvinylidene chloride copolymer film during step (b) isdehydrochlorinated by at least 10%, but not fully dehydrochlorinated.22. The method of claim 16, wherein the thickness is from 10 micrometersto 150 micrometers.
 23. The method of claim 16, wherein thepolyvinylidene chloride copolymer film is formed by melt-extrusion at astretch ratio from 1 to 8.